Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratory-scale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La-Fe-Si-, Heusler- and Fe2P-type compounds.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

The phase‐down scenario of conventional refrigerants used in gas–vapor compressors and the demand for environmentally friendly and efficient cooling make the search for alternative technologies more important than ever. Magnetic refrigeration utilizing the magnetocaloric effect of magnetic materials could be that alternative. However, there are still several challenges to be overcome before having devices that are competitive with those based on the conventional gas–vapor technology. In this paper a rigorous assessment of the most relevant examples of 14 different magnetocaloric material families is presented and those are compared in terms of their adiabatic temperature and isothermal entropy change under cycling in magnetic‐field changes of 1 and 2 T, criticality aspects, and the amount of heat that they can transfer per cycle. The work is based on magnetic, direct thermometric, and calorimetric measurements made under similar conditions and in the same devices. Such a wide‐ranging study has not been carried out before. This data sets the basis for more advanced modeling and machine learning approaches in the near future.

No abstract

Solid-state magnetic refrigeration is a high-potential, resource-efficient cooling technology. However, many challenges involving materials science and engineering need to be overcome to achieve an industry-ready technology. Caloric materials with a first-order transition-associated with a large volume expansion or contraction-appear to be the most promising because of their large adiabatic temperature and isothermal entropy changes. In this study, using experiment and simulation, it is demonstrated with the most promising magnetocaloric candidate materials, La-Fe-Si, Mn-Fe-P-Si, and Ni-Mn-In-Co, that the characteristics of the first-order transition are fundamentally determined by the evolution of mechanical stresses. This phenomenon is referred to as the stress-coupling mechanism. Furthermore, its applicability goes beyond magnetocaloric materials, since it describes the first-order transitions in multicaloric materials as well.under the application of an external field (i.e., electric, magnetic, mechanical stress, or pressure), as illustrated in Figure 1.Recently, the idea of utilizing not just one of these external fields, but a combination of multiple stimuli to transform the caloric material, was proposed, the so-called multicaloric effect. [15][16][17][18][19] In this study, we demonstrate that the caloric effect in materials with a first-order transition is always a consequence of multiple stimuli. Using the example of the three most promising magnetocaloric materials, La-Fe-Si, [20][21][22] Fe 2 P-type, [23][24][25][26] and Heusler alloys, [27][28][29] we develop a model from which the crucial role of mechanical stress is clear. This stress-coupling model does not involve any magnetism and is therefore immediately applicable to all the other caloric effects presented in Figure 1, helping us to understand first-order transitions in general. Cooling Technology

No abstract